Is thermal limitation the primary driver of elevational distributions? Not for montane rainforest ants in the...

Terrestrial ectotherms are likely to be especially sensitive to rising temperatures over coming decades. Thermal limits are used to measure climatic tolerances that potentially affect ectotherm distribution. While there is a strong relationship between the critical thermal maximum (CT_max) of insects and their latitudinal ranges, the nature of this relationship across elevation is less clear. Here we investigated the combined relationships between CT_max, elevation and ant body mass, given that CT_max can also be influenced by body mass, in the World Heritage-listed rainforests of the Australian Wet Tropics. We measured the CT_max and body mass of 20 ant species across an elevational gradient from 350 to 1000 m a.s.l. Community CT_max did not vary systematically with increasing elevation and there was no correlation between elevation and elevational ranges of species. However, body mass significantly decreased at higher elevations. Despite the negative correlation between CT_max and body mass at the community level, there was no significant difference in CT_max of different-sized ants within a species. These findings are not consistent with either the climatic variability hypothesis, Rapoport's rule or Bergmann's rule. Models indicated that elevation and body mass had limited influences on CT_max. Our results suggest that the distribution of most montane ants in the region is not strongly driven by thermal limitation, and climate change will likely impact ant species differently. This is likely to occur primarily through changes in rainfall via its effects on vegetation structure and therefore thermal microhabitats, rather than through direct temperature changes.